Base stations used in wireless telecommunication systems have the capability to receive linear polarized electromagnetic signals. These signals are then processed by a receiver at the base station and fed into the telephone network. In practice, the same antenna which receives the signals can also be used to transmit signals. Typically, the transmitted signals are at different frequencies to the received signals. Receiving signals on two orthogonal polarizations helps to reduce fading caused by multiple reflections at buildings, trees etc.
An array of slant 45. degree polarized radiating elements is constructed using a linear or planar array of crossed dipoles located above a ground plane. A crossed dipole is a pair of dipoles whose centers are co-located and whose axes are (in general) orthogonal. The axes of the dipoles are arranged such that they are parallel with the polarization sense required. In other words, the axis of each of the dipoles is positioned at some angle with respect to the vertical axis of the antenna array.
One problem associated with a crossed dipole configuration is the interaction of the electromagnetic field of each crossed dipole with the fields of the other crossed dipoles and the surrounding structures which support, house and feed the crossed dipoles. As is well known in the art, the radiated electromagnetic fields surrounding the dipoles transfer energy to each other. This mutual coupling influences the correlation of the two orthogonally polarized signals. The opposite of coupling is isolation, i.e., coupling of −30 dB is equivalent to 30 dB isolation. Dual polarized antennas have to meet a certain port-to-port isolation specification.
Another problem associated with antennas in general, is the provision of an antenna element with an appropriate band width performance.
A conventional crossed dipole antenna is shown in U.S. Pat. No. 6,072,839. Six crossed dipole assemblies are mounted in line along a reflector, with a parasitic element located between the inner two dipole assemblies to improve isolation. A disadvantage of parasitic elements is that they disturb the radiation field of the antenna, creating unwanted side lobes and/or decreasing polarization purity.
A crossed-drooping bent dipole antenna is shown in U.S. Pat. No. 6,211,840. In one form the ends of the dipole arms are bent back towards the central axis in a plane parallel to the central axis. In another form the ends of the dipole arms are bent in the same rotational direction out of a plane which includes the central axis.
The bent arms are designed to improve gain and axial ratio at low elevation angles.